Emulator EmulatorJS NES FCEUmm

EmulatorJS + FCEUmm — NES

The NES bundle lives at emulators/jsnes/. The directory name is a legacy artefact — the bundle migrated from bfirsh/jsnes to EmulatorJS + FCEUmm a while ago. The dir was kept to avoid forcing every fs.js link to change. If you're looking at the directory and wondering why an EmulatorJS bundle is named jsnes/, that's why.

Where we started

The first NES bundle used bfirsh/jsnes v2.1.0, a pure-JS NES emulator. It worked, but its audio pipeline used the deprecated ScriptProcessorNode path, which is glitchy in modern browsers — clicks, dropouts, frame-locked audio drift. Long-term that path is going away entirely.

EmulatorJS + the FCEUmm libretro core has a clean AudioWorklet pipeline. Same gain in audio quality the rest of the EmulatorJS bundles got.

The migration

The shape is the same as every other EJS bundle. From the C64 recipe:

• EJS_core = 'nes'
• EJS_pathtodata computed from URL relative to _shared-ejs/ejs/data/
• EJS_startOnLoaded = true
• EJS_DEBUG_XX = true
• FCEUmm core lives at _shared-ejs/ejs/data/cores/fceumm-legacy-wasm.data

Same localStorage ambush, same -legacy- core variant gotcha, same EJS_DEBUG_XX workaround as the VICE family. Refer to Emulator-VICE-family for the long-form explanation of each — they apply identically.

The B/A swap that came along for the ride

bfirsh/jsnes mapped X = A, Z = B — period-correct (A on right, B on left, X is the right-hand key, Z is the left-hand key). EmulatorJS's RetroPad default is the inverse: B (button 0) on x, A (button 8) on z. Period-correct to the physical NES is the opposite direction: A is the right button, so it should be the right-hand key.

Either side has a defensible reading. The deciding factor: users who'd already learned the bfirsh layout. Swap the defaults to preserve muscle memory:

window.EJS_defaultControls = {
  0: {
    0: { value: 'z' },  // B
    8: { value: 'x' }   // A
  }
};

X stays A, Z stays B. Anyone who learned the old build doesn't have to relearn.

A documentation gotcha

When adding new emulator-list claims to README / ATTRIBUTION / Roadmap / File-Structure, the current set of EmulatorJS libretro cores is: FCEUmm + gearcoleco + 4 VICE cores (x64, x128, xvic, xplus4). FCEUmm is easy to forget when listing them — the dir name "jsnes" doesn't suggest a libretro core lives behind it. The PET used to ride along on vice_xpet but was migrated out in June 2026; see Emulator-pet2001-Skibo.

Bundle layout

emulators/jsnes/      ← name is legacy; runtime is EJS+FCEUmm
├── play.html         ← B/A swap + standard EJS bootstrap
├── controls.html
├── games.json
└── games/
    └── *.nes

The FCEUmm core lives at emulators/_shared-ejs/ejs/data/cores/fceumm-legacy-wasm.data.

Related

• Emulator-VICE-family — same EJS bootstrap pattern, all the EJS gotchas explained in full
• Emulator-gearcoleco-ColecoVision — sibling EJS bundle with controller overrides
• Emulators — index